BONDERIZED STEEL STRIP: THE MULTITALENTED STEEL MATERIAL WITH A ZINC PHOSPHATE COATING

The special feature of our so-called bonderized steel strip is the zinc phosphate which forms a coating around the material. Chemically bonded to the strip surface, it is truly a multitalented layer that can be used as the basis for further processing and finishing steps. For example, in combination with lubricant, it ensures that the solid material exhibits excellent formability, or can serve as an adhesive primer for a wide range of surface finishes. Thanks to precisely calibrated capillary sizes, this zinc phosphate layer is extremely versatile.

The zinc phosphating process, also known as bonderizing, forms a crystalline layer on the surface of the strip that is interspersed with cavities and capillaries. With its needle-shaped structure, it enlarges the actual surface of the strip, creating an optimum surface structure for further processing. During the zinc phosphating process, the coating thickness, capillary size, and surface roughness can be precisely calibrated. The special feature here is that the zinc phosphate coating is chemically bonded to the strip, offering a wide range of options for subsequent surface finishing:

• Treatment with soaps
• Treatment with grease, oil, or other lubricants
• Application of varnishes and organic coatings
• Application of non-film-forming organic sealants
• Application of inorganic or organic sealants

Lubricating layer for complex forming processes

A number of products made of cold rolled steel strip are rather small, but have complex shapes and are subject to high levels of stress, such as bucket tappets, needle bearing sleeves, tension and idler pulleys, and components for power saws and drills. The challenge in this context is that for their specific area of application, some components also require a material with increased tensile strength. Creating their complex shapes requires a number of forming processes, however, for which increased material strength is not always ideal. Zinc phosphate coated steel strip offers the perfect solution to these seemingly contradictory requirements. The material’s surface, which in this case is coated with a special lubrication system, significantly reduces friction during the forming processes, thus reconciling these two opposing needs.

Baroudi Zelit, responsible for bonderized coated steel strip production at Waelzholz as Head of Quality Assurance, explains the principle behind the strip’s lubricant carrier layer: “During the phosphatization process, we specially finish the surfaces of cold rolled steel strip grades. In this context, the material is coated with layers of zinc phosphate, zinc stearate, and a reactive soap in different production steps. The result is a lubrication system that is firmly bonded to the material.”

The standard lubrication also makes a higher number of strokes possible in the forming processes and helps reduce wear on the forming tool. At the same time, the zinc phosphatized strip not only withstands the mechanical and thermal stresses of complex forming operations, but also maintains its lubrication performance over multistage forming processes due to the increase in surface area. This is due to the fact that the lubrication system is not only applied to the surface, but becomes embedded in the capillaries over the entire thickness of the coating.

Consistently high quality on seven zinc phosphate coating lines worldwide

The multitalented bonderized steel strip, with its zinc phosphate coating, is being designed to meet each customer’s individual requirements. For a customer in the automotive industry, for example, the strip is specially brushed to achieve maximum adhesion and precision during deep-drawing processes.

“Together with our customers, we define how the zinc phosphate coated steel strip’s properties need to be calibrated. In quite a few cases, we also develop custom solutions,” reports phosphate coated steel strip expert Zelit. “With our expertise, we always consider not just the requirements of the material’s subsequent use, but also the customer’s unique production processes. Machine efficiency, low tool wear, and consistent quality are the focus in this context.”

Once developed and finalized with the customer, the zinc phosphate coated steel strip can be produced identically at different Waelzholz locations worldwide. Waelzholz currently operates seven fully harmonized bonderizing lines at locations in Brazil, China, and Germany. This means that this multitalented zinc phosphatized steel strip can be supplied to customers all over the world with a consistently high material quality.